The present invention is directed, in general, to magnetic devices and, more specifically, to a magnetic device employing a winding structure spanning multiple boards and method of manufacture thereof
Magnetic devices, such as inductors and transformers, are employed in many different types of electrical circuits, such as power supply circuits. In practice, most power magnetic devices are fabricated of one or more windings formed by an electrical member. The electrical member may be a wire of circular or rectangular cross section or a planar conductor wound about or mounted to a bobbin composed of dielectric material, such as plastic. In some instances, the electrical member is soldered to terminations on the bobbin. Alternatively, the electrical member may be threaded through the bobbin for connection directly to a metallized area on a circuit board. A magnetic core is affixed about the bobbin to impart a greater reactance to the power magnetic device.
As with other types of electronic components, there is a trend in the design of power magnetic devices toward achieving increased power and volumetric density and lower device profile. To achieve higher power, the resistance of the power magnetic device must be reduced, typically by increasing the cross-sectional area of the electrical member forming the device windings, or by simply reducing the electrical path length of the device. To increase the density of the power magnetic device, the bobbin is usually made relatively thin in the region constituting the core of the device to optimize the electrical member resistance. Conversely, the remainder of the bobbin is usually made relatively thick to facilitate attachment of the electrical member to the bobbin terminals or to facilitate attachment of terminals on the bobbin to a circuit board. As a result of the need to make such a bobbin thin in some regions and thick in others, the bobbin is often subject to stresses at transition points between such thick and thin regions.
Another problem associated with present-day power magnetic devices is the lack of co-planarity of the device terminations. Because of the need to optimize the winding thickness of the power magnetic device to provide the requisite number of turns while minimizing the winding resistance, the thickness of the electrical member forming each separate winding of the device is often varied. Variation in the winding thickness often results in a lack of co-planarity of the device terminations, an especially critical deficiency when the device is to be mounted onto a surface of a substrate, such as a printed wiring board (PWB).
Power magnetic devices, suitable for attachment to a substrate such as a PWB, may include at least one sheet winding having a pair of spaced-apart terminations. The sheet winding terminations and lead portions, together with at least a portion of the sheet windings, may be surrounded by a molding material. A magnetic core may surround at least a portion of the plurality of individually insulated sheet windings to impart a desired magnetic property to the device. Thus, a bobbin-free, surface-mountable power magnetic device that overcomes some of the deficiencies discussed above therefore represents an advance over the previously-described power magnetic devices (See for instance, U.S. Pat. No. 5,345,670, entitled xe2x80x9cMethod of Making a Surface Mount Power Magnetic Device,xe2x80x9d issued on Sep. 13, 1994, to Pitzele, et al. which is incorporated herein by reference).
In certain of these approaches, the sheet windings form a portion of a PWB that attaches to another PWB. The device leads often extend substantially from the device footprint and therefore increase the area of the substrate required to mount the device. In fact, extended leads can add 30% to the footprint or 50% to the volume of the magnetic device. Also, termination co-planarity may require that either the aforementioned devices be molded in a lead frame (requiring additional tooling and tighter tolerances) or that the leads be staked in after molding (requiring an additional manufacturing operation). Additionally, the separate PWB and outer molding compound, if employed for electrical isolation and thermal conductivity, adds both volume and cost and raises the device profile.
Alternately, the sheet windings may be part of a multilayer circuit such as an FR-4 board (manufactured by Key Grant, of Fountain Vally, Calif.) thereby eliminating the separate PWB altogether (See, for instance, , U.S. patent application Ser. No. 08/940,557, entitled xe2x80x9cPower Magnetic Device Employing Leadless Connection to a Printed Circuit Board And Method of Manufacture Thereof,xe2x80x9d filed May 4, 1995, to Pitzele, et al., and U.S. patent application Ser. No. 08/940,672, entitled xe2x80x9cPost-mountable Planar Magnetic Device And Method of Manufacture Thereof,xe2x80x9d filed May 4, 1995, to Pitzele, et al., which are incorporated herein by reference). There are instances, however, where employing a separate multilayer circuit to accommodate the windings is not the design of choice.
In either case, the separate PWB or multilayer circuit must have enough layers to accommodate the total number of windings needed. A transformer typically consists of a minimum of one primary winding and one secondary winding. This results in a minimum of a two layer board, given a transformer turns ratio of 1:1. Multiple outputs and other turns ratios may easily result in a transformer requiring as many as 16 layers. Current circuit PWBs typically accommodate only four layers economically, but may include eight layers (or more) at over twice the cost of four layers. If a PWB requires two to four layers to accommodate the other electronic components, adding additional layers to accommodate the need for more sheet windings in a magnetic device becomes cost prohibitive and reliability of the circuit may become an issue. This also makes the circuit board unique to only one transformer ratio.
Accordingly, what is needed in the art is an improved way to accommodate the need for additional sheet winding layers that overcomes the deficiencies in the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a magnetic device and a method of manufacturing the same. In one embodiment, the device includes: (1) a magnetic core and (2) a winding structure located proximate the magnetic core. The winding structure includes:(2a) a multilayer main circuit board containing an interconnected first plurality of winding layers, (2b) a multilayer overlay board located proximate the main circuit board and containing an interconnected second plurality of winding layers and (2c) conductors coupling the first and second pluralities of winding layers together to cause the first and second pluralities of winding layers to function cooperatively as windings for the magnetic device.
The present invention, in one aspect, introduces the broad concept of judiciously distributing the winding structure associated with a magnetic device to allow a more advantageous packaging or utility of the magnetic device. Those skilled in the pertinent art will understand that the term xe2x80x9cmagnetic devicexe2x80x9d includes both transformers and inductors. The present invention encompasses not only these devices, but all forms thereof and any later-developed magnetic devices so structured.
In one embodiment of the present invention, the magnetic core includes a first portion coupled to the main circuit board and a second portion coupled to the overlay board. Of course, the magnetic core may be one piece. Any appropriate magnetic material may be used for the magnetic core, and it may be constructed in a variety of appropriate forms, including an E-core structure or other types having a gap or being gapless.
In one embodiment of the present invention, the magnetic core is surface-mounted to the main circuit board and the overlay board. The magnetic core may be glued to either the main circuit board or the overlay board or it may be glued to each as the application may dictate. Additionally, a through-hole mounting structure may also be used with either or both of the main circuit and overlay boards.
In one embodiment of the present invention, the overlay board is oriented parallel to the main circuit board. In an alternate embodiment, the overlay board may be perpendicular or offset to the main circuit board. Of course, other orientations are possible and well within the broad scope of the present invention.
In one embodiment of the present invention, the overlay board is affixed directly to the main circuit board. Alternately, an intervening structure or gap may be used. Additionally, the overlay board and the main circuit board may each consist of four layers. Of course, other numbers of layers are possible and well within the broad scope of the present invention.
In one embodiment of the present invention, the conductors are selected from the group consisting of: (1) corresponding conductive vias located in each of the overlay board and the main circuit board, (2) at least one conductive post located on one of the overlay board and the main circuit board and (3) a connector coupled to an edge of the overlay board. Those skilled in the art will readily perceive that other conventional or later-discovered structures for interconnecting boards or devices fall within the broad scope of the present invention.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.